1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus which purifies exhaust gases exhausted from an internal combustion engine, and temporarily adsorbs hydrocarbons within exhaust gases upon start of the internal combustion engine.
2. Description of the Prior Art
The type of conventional exhaust gas purifying apparatus for an internal combustion engine mentioned above is known, for example, from Laid-open Japanese Patent Application No. 2000-310113. The exhaust gas purifying apparatus disclosed therein comprises a pair of upstream and downstream three-way catalysts in an exhaust pipe of an internal combustion engine. An inner main exhaust passage and an annular bypass passage around the main exhaust passage are formed between the two three-way catalysts within the exhaust pipe. The bypass passage has a passage area smaller than the main exhaust passage, and is filled with a hydrocarbon adsorbent. A switching valve is also provided at the inlet port of the main exhaust passage for opening and closing the main exhaust passage.
For controlling the switching valve, it is determined whether or not the following three conditions are met after the engine is started:
1) whether or not a cooling water temperature of the engine detected by a water temperature sensor is lower than a predetermined temperature;
2) whether or not the amount of intake air detected by an air flow meter is smaller than a predetermined amount; and
3) whether or not a time elapsed after the start is shorter than a catalyst activation time which is determined in accordance with the cooling water temperature.
When the three conditions are all met, the switching valve is fully closed on the assumption that the downstream three-way catalyst has not been activated. In this state, exhaust gases passing through the upstream three-way catalyst are entirely passed to the bypass passage, so that hydrocarbons within the exhaust gases are adsorbed by the adsorbent filled in the bypass passage. Then, the exhaust gases flow into the downstream three-way catalyst, thereby preventing hydrocarbons from being emitted to the atmosphere. On the other hand, when any of the three conditions is not met, the switching valve is fully opened on the assumption that the downstream three-way catalyst has been activated. In this state, a majority of exhaust gases pass through the main exhaust passage, which has a larger passage area, and subsequently flows into the downstream three-way catalyst for purification through its oxidation/reduction actions.
However, the conventional exhaust gas purifying apparatus determines whether the downstream three-way catalyst is activated after the start of the engine based on the cooling water temperature, amount of absorbed air, and time elapsed after the start, which are used as parameters, so that the exhaust gas purifying apparatus may fail to make appropriate determination, resulting in the inability to switch the switching valve at an appropriate timing. For example, since the downstream three-way catalyst is located substantially away from the engine body for which the cooling water temperature is detected, the temperature of the downstream three-way catalyst rises with a delay from the cooling water temperature. As such, the cooling water temperature does not always match the temperature of the downstream three-way catalyst in rising timing, behavior and the like. Therefore, the cooling water temperature may not exactly reflect the actual temperature state of the downstream three-way catalyst, i.e., whether it is activated.
To solve the disadvantage as mentioned above, the temperature of the downstream three-way catalyst may be directly detected by a temperature sensor for use as a parameter instead of the cooling water temperature. With this strategy, however, the activation of the three-way catalyst cannot either detected with high accuracy because the temperature sensor generally has a responsibility too low for use with the downstream three-way catalyst which is activated in a relatively short time after the start of the engine, and also because the temperature sensor experiences difficulties in detecting the temperature at the center of the downstream three-way catalyst, which is critical for evaluating whether or not the three-way catalyst is activated, in a temperature distribution of the three-way catalyst which can readily vary when the temperature rises in such a short time.
Also, since the temperature rising rate of the downstream three-way catalyst depends on a particular operating condition after the start of the engine (for example, when the vehicle is idled after the start, and when the vehicle is launched immediately after the start), the time elapsed after the start does not either reflect exactly an actual activated state of the downstream three-way catalyst. Further, in regard to the amount of intake air, since a detection value detected every predetermined time is compared with a predetermined value, the conventional exhaust gas purifying apparatus will erroneously determine that the downstream three-way catalyst has been activated if the amount of intake air instantaneously increases. From the result of the foregoing analysis, the conventional exhaust gas purifying apparatus cannot set a timing at which the switching valve is switched to the main exhaust passage appropriately in response to a transition of the downstream three-way catalyst into activation. Consequently, if the switching valve is switched at a timing too early, exhaust gases will flow into the inactivated downstream three-way catalyst, so that hydrocarbons will be emitted to the atmosphere to exacerbate the exhaust gas characteristic. On the other hand, if the switching valve is switched at a timing too late, exhaust gases will flow into the downstream three-way catalyst with a delay, though it has been already activated, thereby failing to effectively utilize the purifying performance.
The present invention has been made to solve the problems as mentioned above, and it is an object of the invention to provide an exhaust gas purifying apparatus for an internal combustion engine which is capable of switching a switching valve at an optimal timing in accordance with an actually activated state of a catalyzer to achieve an optimal exhaust gas characteristic.
To achieve the above object, the present invention provides an exhaust gas purifying apparatus for an internal combustion engine for purifying exhaust gases of the internal combustion engine, and temporarily adsorbing hydrocarbons within exhaust gases upon start of the internal combustion engine. The exhaust gas purifying apparatus is characterized by comprising a catalyzer disposed in an exhaust system of the internal combustion engine for purifying exhaust gases; an adsorbent filled in a second passage circumventing a first passage in the exhaust system for adsorbing hydrocarbons within exhaust gases; a switching valve operable to switch between an open position for opening the first passage and a close position for closing the first passage; start-time temperature state detecting means for detecting a temperature state of the exhaust system upon start of the internal combustion engine; post-start exhaust gas calory calculating means for calculating the calory of exhaust gases discharged after the start of the internal combustion engine; and switching valve driving means for driving the switching valve to the close position upon start of the internal combustion engine, and driving the switching valve to the open position in accordance with the detected start-time temperature state of the exhaust system, and the calculated post-start exhaust gas calory.
According to this exhaust gas purifying apparatus for an internal combustion engine, the switching valve is driven to the close position to close the first passage upon start of the internal combustion engine, thereby forcing exhaust gases to flow into the second passage which circumvents the first passage. In this way, hydrocarbons within the exhaust gases are adsorbed by the adsorbent filled in the second passage, preventing the emission of the hydrocarbons to the atmosphere.
Subsequently, the switching valve is driven to the open position to open the first passage in accordance with a start-time temperature state of the exhaust system, and post-start exhaust gas calory. In this way, exhaust gases are guided to flow into the first passage, and the adsorbent finishes adsorbing hydrocarbons. The exhaust gases are purified by the activated catalyzer before they are emitted to the atmosphere. The start-time temperature state is a parameter indicative of a temperature state of the exhaust system and the catalyzer provided therein upon start of the internal combustion engine, while the post-start exhaust gas calory is a parameter indicative of the calory applied to the exhaust system including the catalyzer from exhaust gases after the start. Therefore, a combination of these two parameters exactly reflect the temperature state, i.e., activated state of the catalyzer after the start. Also, the activated state of the catalyzer is evaluated based on the parameter indicative of the temperature state only upon starting in the foregoing manner, and is evaluated based on the exhaust gas calory used as a parameter after the start based on the temperature state. It is therefore possible to highly accurately determine the activated state of the catalyzer while avoiding the inaccuracy which would be resulted when the activated state is determined using the result of detection by a temperature sensor after the start. Consequently, the switching valve can be driven to the open position at an optimal timing immediately after the catalyzer is actually activated, thereby achieving an optimal exhaust gas characteristic.
Preferably, in the exhaust gas purifying apparatus for an internal combustion engine, the start-time temperature state detecting means includes stop-time temperature detecting means for detecting the temperature of the exhaust system at the preceding stop of the internal combustion engine; and inoperative time measuring means for measuring an inoperative time from the preceding stop to the current start of the internal combustion engine, wherein the start-time temperature state detecting means is configured to find the start-time temperature state of the exhaust system in accordance with the detected stop-time temperature of the exhaust system and the measured inoperative time.
The temperature in the exhaust system upon stop of the internal combustion engine depends on an operating condition including an operating time of the internal combustion engine until the stop, and the temperature in the exhaust system after the stop varies from this stop-time temperature in accordance with a time elapsed from the stop. Thus, according to this preferred embodiment of the exhaust gas purifying apparatus, the start-time temperature state in the exhaust system can be highly accurately detected in accordance with the preceding operating condition of the internal combustion engine, and the duration in which the engine has been inoperative. Thus, the activated state of the catalyzer can be determined with higher accuracy in accordance with the start-time temperature state of the exhaust system, so that the switching valve can be more appropriately switched in accordance with the activated state of the catalyzer.
Preferably, in the exhaust gas purifying apparatus for an internal combustion engine, the start-time temperature state detecting means further includes an ambient temperature detecting means for detecting the ambient temperature around the internal combustion engine, wherein the start-time temperature state detecting means is configured to find the start-time temperature state of the exhaust system in accordance further with the detected ambient temperature.
The temperature in the exhaust system after the internal combustion engine is stopped varies depending not only on the length of elapsed time after the stop but also on the ambient temperature around the internal combustion engine, and varies at a larger rate, for example, when there is a larger difference between the temperature in the exhaust system upon stop of the internal combustion engine and the ambient temperature. Thus, according to this preferred embodiment of the exhaust gas purifying apparatus, the ambient temperature around the internal combustion engine is employed as an additional parameter to more accurately detect the start-time temperature state of the exhaust system, thereby making it possible to more appropriately determine whether the catalyst is activated and switch the switching valve.